This invention relates to a system for providing cold injectate to a thermodilution cardiac output measuring catheter. Thermodilution catheters have been used to determine cardiac output and these catheters are typically small diameter balloon types equipped with distal temperature sensing means and a lumen opening a short distance proximal to the temperature sensor for introduction of a low-temperature liquid injectate into the blood stream. The displacement of temperature resulting from the introduction of the low temperature injectate is sensed by the temperature sensing means, usually a thermistor. Such systems include a suitable supply of sterile injectate, usually 5% Dextrose in water or normal saline, and a multi-lumen catheter having at least one lumen with a thermistor in it, another for injectate, and yet another for a balloon. The injectate lumen has a proximal opening or port which is approximately 28 centimeters from the distal tip through which the injectate is infused. There is a thermistor distal therefrom and a balloon even more distal and finally a distal lumen.
The magnitude and duration of the temperature displacement over time can be used to compute the blood flow rate for a measure of a patient's cardiac output. U.S. Pat. No. 3,995,623 shows a typical thermodilution catheter. The blood flow rate is computed from the displacement of blood temperature according to the Stewart-Hamilton dilution equation for a thermal indicator as described in U.S. Pat. No. 3,987,788. As per that prior patent, numerical values are used for a computation constant, blood temperature, and injectate temperature. The computation constant is derived from the nature of the injectate, the volume of the injectate and a correction factor for the rise in temperature of the injectate as it passes through the lumen of the catheter to the injectate orifice.
Systems such as the Co-Set.TM. from American Edwards Laboratories, Santa Ana, California require that the injectate be supplied by a syringe filled from a coil of tubing placed in a container filled with crushed ice. The ice covers the cooling coil of tubing and water is used to affect heat transfer between the tubing and the ice. The container is insulated styrofoam. Difficulties with thermodilution cardiac output monitoring include time wasted in loading the syringe, uncertainty of the syringe injectate temperature notwithstanding thermistors provided for that purpose, inconsistent techniques which are a consequence of the clinicians misuse or unfamiliarity with the apparatus and concern about air bubbles in the system as a consequence of the filling of the syringe and then injection with the syringe.
U.S. Pat. No. 3,293,868 issued Dec. 27, 1966 to F. A. Gonzalez for "Fluid Cooling Apparatus" describes a device having a flat plate with sinuous-shaped upstanding fins which form a channel for holding a length of flexible tubing through which blood or the like can be passed for heating or cooling. The plate is in contact with a number of spaced thermocouples which operate by virtue of the thermoelectric effect to heat or cool the plate depending upon the direction current is passed through the thermocouples. A rotary blower may be further used to remove heat from the fluid. The device is designed to be used in a generally horizontal position with the blood or other fluid being forced through the tubing by a pump or the like. The apparatus lacks an automatic injection system for easily and accurately delivered volumes of fluid and a simple disposable heat exchanger.
The Shah et al U.S. Pat. No. 4,532,414 shows an inline fluid warmer for heating parenteral fluids such as blood. The warmer includes an enclosure containing a heated plate having a sinuously-shaped groove configured to accept and hold the length of supply tubing in heat transfer relationship with the plate. Suitable temperature controls are provided to regulate the heat and keep the blood at the preferred level of heating. Good heat transfer in a disposable heat exchanger is not taught.
Inconsistencies in injectate temperature and variations in injection technique, such as injection speed or smoothness of the clinician can cause erroneous cardiac output measurements. These measurements could possibly result in inadvertent misdiagnosis and treatment which can be potentially fatal. Also, injection of air bubbles into the pulmonary vascular system is another potentially fatal situation due to the likelihood of pulmonary embolism.
Motor actuated syringes have been subject of numerous prior art patents, for example, U.S. Pat. Nos. 3,336,925; 3,156,236; 3,335,724; 2,896,621; 2,457,977; 3,313,291; 2,602,446; 2,498,672; and 3,584,623. However, none of these devices include an easily adjustable mechanism for accurately varying the amount of volume injected.